Dielectric constant, dielectric loss and DC resistivity are critical properties governing the electrical behavior of insulating materials (dielectric materials) utilized in electronic applications, including electronic packaging applications. In many instances, a low dielectric loss material is desirable, because low dielectric loss reduces absorption and distortion of electrical signals traveling throughout the package. However, there are instances where a high dielectric loss insulating material is desirable. Clearly, the ability to vary and control dielectric loss in electronic packaging materials would potentially be of critical importance.
Dielectric Constant (K), the real part of the relative complex permitivity of a material, is the ratio of the capacitance of a given configuration of electrodes with the material as a dielectric to the capacitance of the same electrode configuration with a vacuum (or air for most practical purposes) as the dielectric. For high speed digital circuitry, the dielectric constant is important, because the transmission speed of a pulse in a circuit in contact with the dielectric is inversely proportional to the square root of this quantity. Thus, the lower the dielectric constant, the higher the transmission speed that can be maintained. Dielectric constant also affects crosstalk between adjacent signal lines.
Dielectric loss is electrical energy transformed into heat in a dielectric subjected to a changing electric field. Dielectric loss is usually expressed in terms of loss index or dissipation factor.
Loss Index (K") of a material is the magnitude of the imaginary part of its relative complex permitivity and is proportional to the power dissipated in the material.
Dissipation Factor (D) (tan .delta. or loss tangent) is the ratio of the loss index to the dielectric constant (K"/K'). It is also the tangent of the loss angle, .delta.. The quantity 2.multidot..omega..multidot.D (where .omega. is the angular frequency of the applied AC field) is the time average power dissipated in a material divided by the time average stored electric energy in that material. Dissipation Factor thus gives a measure of the efficiency by which electric energy may be stored in a material medium, and must be kept low for efficient storage or transmission of energy.
While dielectric constant affects the speed of an electronic signal, dielectric loss affects absorption of the signal. An electronic signal, being a "packet" of frequencies, is subject to selective absorption of given frequencies, resulting in a distortion of the signal. Low loss dielectric materials, thus having a low dissipation factor, will lower the absorption and/or distortion of the signal. For a dielectric material used in a capacitor or resonator, if the material exhibits high dielectric loss, then some of the energy being stored is lost. On the other hand, a high dielectric loss material stabilizes "ringing", that is, reduces voltage oscillation at the leading and trailing edges of rectangular voltage pulses.
Higher dielectric constant materials in graded structures and higher dielectric loss materials are also used at the edges of metallized regions carrying high voltages to reduce voltage gradients and AC corona.
Dielectric loss is typically a strong function of both frequency and temperature. At low frequency (typically less than a few megahertz) and/or high temperature, interfacial polarization effects (caused by charge carriers that are free to move over greater than atomic distances) dominate dielectric loss. At radio frequencies (RF), microwave frequencies and infrared frequencies, dipolar interactions dominate dielectric loss.
Dielectric Strength is the maximum electric field that a dielectric can withstand, without resulting in a voltage breakdown.
A corresponding property is Breakdown Voltage, that is, the voltage necessary to cause the passage of appreciable electric current without a connecting conductor, commonly used to express the voltage at which an insulator fails to withstand the voltage and ceases to behave as an insulator.
Aluminum nitride (AlN) ceramic, particularly in the form of sintered bodies, is an insulator, or dielectric material, that has great potential for electronic applications, particularly electronic packaging. Aluminum nitride in a dense, sintered form possesses a high thermal conductivity, greater than alumina (a conventional, commercial electronic packaging dielectric) and approaching that of beryllia (an electronic packaging dielectric material having toxicity considerations).
Further, aluminum nitride possesses a coefficient of thermal expansion close to that of silicon and gallium arsenide, a high flexural strength, a high hardness, chemical inertness and nontoxic behavior.
Due to its high thermal conductivity, aluminum nitride is usually used in applications where high heat dissipation is required and where the ceramic would be required to operate at elevated temperature. Aluminum nitride substrates are useful where high heat dissipation is required in a microelectronic package, such as in a multilayer metal-ceramic package for high power devices. For example, in microelectronic applications the current required operating temperature ranges from approximately 75 to 200.degree. C. The required temperatures could increase substantially when semiconductors currently under intensive study (such as silicon carbide) come into commercial use.
Aluminum nitride ceramics are also useful for other electronic applications which require heat tolerance and electrical insulating properties. An example would be in electrostatic chucks, where the operating temperature range is 400-500.degree. C.
Prepared from aluminum nitride powders, in order to achieve suitable properties the ceramic must achieve a certain density, at least about 90%, preferably greater than or equal to about 95%, of theoretical. Aluminum nitride decomposes below the temperature required to sinter it to maximum density. However, densification can be achieved at lower temperatures by the use of sintering aids.
Sintering aids liquify at temperatures below the decomposition and pure compound sintering temperatures for the ceramic, and promote densification of the ceramic grains. At the later stages of the liquid sintering cycle, microstructure is refined via grain growth and coalescence processes. Microstructure and grain size can have an effect on dielectric properties.
Sintering aids also function to increase thermal conductivity of the sintered aluminum nitride body by gettering oxygen from the aluminum nitride powder. Thus, an effective sintering additive forms a liquid capable of dissolving and reprecipitating aluminum nitride, without depositing significant amounts of oxygen in the densifying ceramic.
All commercially available aluminum nitride powders contain oxygen as an impurity. This oxygen primarily takes two forms in the powder, as a native alumina coating on each of the powder particles, and as dissolved oxygen impurity within the crystalline lattice of the aluminum nitride particles. A minor amount will be tied up as an oxide of any metal impurities which may be present.
At a given sintering temperature, only a certain amount of oxygen, primarily from native alumina and secondarily from other sources, will be available for reaction (hereinafter "available oxygen"). A certain amount of oxygen will remain bound up in the aluminum-nitrogen crystalline lattice, and thus a true stoichiometric ratio of aluminum to nitrogen is not achieved. The resulting sintered, polycrystalline aluminum nitride exhibits properties, including dielectric properties, which deviate from those which would be exhibited by pure, single crystal AlN.
Sintering aids for AlN which have been disclosed in the art include Group IIa, Group IIIa, and/or rare earth compounds, including calcia and yttria, among others. Resulting AlN sintered bodies are disclosed to also contain second phase materials, including alkaline earth-aluminates, Group IIIa-aluminates, rare earth-aluminates, ternary oxides of aluminum and at least two sintering aid metals, and AlON.
U.S. Pat. No. 4,618,592 discloses the use of sintering aids for aluminum nitride which are at least one metal element selected from alkaline earth metals, lanthanum group metals and yttrium or a compound thereof. The patent further discloses that bound oxygen and impurity content affect the light transmitting property of the resulting sintered body, and that preferably, the bound oxygen content is at most 1.5% by weight.
Conventional aluminum nitride ceramics possess a dielectric constant of between 8.4 and 9 as compared to 10.1 for alumina, and a dissipation factor, measured at room temperature at a frequency of 1 KHz, of between 0.001 and 0.01. These materials typically have been reported to have a direct current (DC) electrical resistivity of 10.sup.14 ohm-cm. Heretofore, it has not been known what functionality within the AlN controls the dielectric loss and DC resistance.
Dielectric properties of aluminum nitride materials have been modified by the addition of different materials to the ceramic, such as transition metal addition as reported in Kasori et al. "Effects of Transition Metal Additions on the Properties of AlN" J. Am. Ceram. Soc. 77 [8] 1991-2000 (1994), or blending of dielectric materials as disclosed in U.S. Pat. Nos. 4,796,077 and 4,960,734.
The layering of various insulating and non-insulating materials has also been shown to affect dielectric properties, including dielectric breakdown voltage, as disclosed in U.S. Pat. No. 4,783,368. Aluminum nitride has been disclosed in U.S. Pat. No. 5,070,046 as a ceramic filler for dielectric compositions, such as borosilicate glass and cordierite glass, which exhibit low dielectric constant and low dielectric loss.
U.S. Pat. No. 5,320,990 discloses the development of thermal conductivity of sintered aluminum nitride of greater than 200 W/M-K by sintering aluminum nitride powder and a powdered sintering aid in the presence of nitrogen gas, including heating to the sintering temperature at a rate of greater than 0.degree. C. to about 6.degree. C. per minute, maintaining at the sintering temperature to obtain greater than 95% theoretical density, and cooling the sintered body in a vacuum or an inert gas to 1400.degree. C. at a rate of greater than 0.degree. C. to about 6.degree. C./minute, before cooling to ambient. This document disclosed that a prior art patent, No. 4,778,778, taught the heating of a compacted AlN body at no more than 250.degree. C. per hour and the cooling of the sintered AlN at a rate of no more than 300.degree. C./per hour, preferably 100.degree. C.-275.degree. C./hr.